1. Technical Field
The present invention relates to a method and arrangement of detecting a number of photons in an x-ray detecting arrangement having a number of spaced apart sensors, wherein the detected photons indirectly create an amount of free charges proportional to the photon energy.
2. Background Information
Although the technique of x-ray imaging was discovered a long time ago, imaging systems of today still use the following simple procedure. Photons from a spectrum of energies are passed through an object and then detected. If a massive body such as a tumor is included in this object, a shadow (i.e., fewer photons) will fall on the detector, creating an image. Obviously, stochastic fluctuations occur in the number of photons that pass through the tissues. A sufficient amount of radiation is needed so that these fluctuations are small relative to the difference in the expected number of photons passing through the different tissues.
Preferably, the most efficient use of the detected photons is made so as to minimize the patient dose. Better detectors have been created for this purpose, with less radiation containing information about the object escaping undetected. However, very little attention has been drawn to the possibilities of increasing or improving efficiency by using information about the energy spectrum after the object. Digital systems open up new possibilities concerning this idea.
U.S. Pat. No. 5,665,969 to Beusch (xe2x80x9cthe ""969 patentxe2x80x9d) describes an x-ray detector, designed to operate as an imaging spectrometer for imaging of a subject. The x-ray detector measures energy of individual x-ray photons in each of a plurality of pixels in the x-ray detector. The pixels of the x-ray detector are readout at a rate such that the likelihood of arrive of more than one x-ray photon in each pixel during a readout period is negligible. Because x-ray photons with different energy levels will create different magnitude responses in the x-ray detector, the measurements made by the x-ray detector can be weighted according to the energy level of the detected x-ray photons. Thus, responses due to noise or x-ray photons which contribute little or no x-ray attenuation information can be discarded or weighted to eliminate or reduce their effect on any resulting image. Conversely, measurements due to x-ray photons which provide significant attenuation information can be weighted significantly.
According to the ""969 patent, the optimal energy weighting one should use is the theoretical optimal one, that is approximately proportional to the negative third power of energy. There is nothing mentioned about charge shares and the optimal weight curve which can be used in reality. To be able to weight the photons with respect to the information content in a realistic way for semiconductor detectors, the signal sharing between the detector pixels must be considered. This has not been obvious until now. The ""969 patent does not consider this possibility. If charge sharing is used in this case for energy weighting, the resultant image will be deteriorated, especially in case of mammography. The suggested method works for detectors having large pixels, in which charge sharing can be neglected and hence this is not a problem. In case of small detectors with spatial resolution the charge sharing will affect the output signal.
In traditional detectors, the signal is usually integrated for each pixel and each individual photon is not considered. In these types of detectors, normally used in hospitals and x-ray examination of material, the charge sharing between the pixels is positive, which increases the signal quality. It is also inherited in the detector structure which is not considered in signal processing. However, for photon-counting detectors, the signal sharing causes problems.
Examples of detectors, on which the present invention can be applied to, are disclosed in U.S. Pat. No. 4,937,453 to Nelson (xe2x80x9cthe ""453 patentxe2x80x9d) and Swedish Patent Application No. 9900856-7. The ""453 patent discloses a method and apparatus for detecting x-ray radiation in a radiographic imaging context using so-called xe2x80x9cedge-onxe2x80x9d detectors. It is particularly useful in conjunction with slit and slot scan radiography. In accordance with this invention, detectors are constructed and arranged such that substantially all of the energy from an x-ray to be detected is discharged in the detector. In this way a detector is provided which provides a direct electronic read out, high x-ray stopping power and high spatial resolution while obtaining good signal collection efficiency without the use of excessively high voltage levels. In the preferred embodiment, solid-state x-ray detectors are constructed such that the thickness of the detector along the direction of incident x-rays is long enough that substantially all of the x-ray energy is discharged in the detector.
Swedish Patent Application No. 9900856-7 refers to a method of obtaining improved radiographic images consisting of orienting a semiconductor radiation detector. The orienting step comprises a selection of an acute angel between the direction of incident radiation and a side of the detector such that the incident radiation mainly hits the side.
FIG. 1 is a schematic illustration of a detector 100 comprising a semi-conducting substrate 110 and spatially arranged sensor or electrode strips 120. Common for these detectors is that stripes of sensors are arranged spaced from each other on a silicon substrate and the x-rays incident onto both the sensors and the space between them.
The article Marks, D. G. et al, xe2x80x9cA 48xc3x9748 CdZnTe array with multiplexer readoutxe2x80x9d, Nuclear Science, IEEE Transactions, vol. 43, issue 3, part 2, June 1996, pp. 1253-59, describes charge spreading in an array of pixels in an x-ray detector on a single substrate. A method of summing nearest-neighbor pixels is disclosed. The photons are only weighted with regard to the amount of charge. The photon energy is re-created with respect to a certain level. Energy levels above a threshold value are not weighted.
The present invention enhances the prior art methods by means of a simple but yet efficient arrangement. In doing so, the present invention provides a novel method and arrangement for detecting and analyzing x-rays in an efficient and accurate manner.
The present invention also reduces the effects of charge sharing and trapping in a photon charge detector arrangement. The invention detects photon transmissions through a tissue and amplifier the contrast through weighting.
A method is provided for enhancing contrast information from an x-ray detecting arrangement when detecting a number of photons in the arrangement. The method includes providing at least two adjacently arranged sensors on one substrate. Each sensor has a corresponding output signal, each of which can be influenced due to shared charge from a photon detected in one of the adjacent sensors. The detected photon indirectly creates an amount of free charges proportional to the photon energy. The influence on the signal is considered by weighting the photon with respect to possible photon charge-share between the at least two adjacent sensors.
The method further includes the step of disregarding the smaller signal, or adding together two signals when signals from the sensors appear within a small time window on at least two neighboring strips.
The method further includes the step of providing an optimal weighting curve by calculating distributions of charge sharing for each energy bin in a photon spectrum that enters the detector and using an optimal theoretical weight curve for said spectrum. Moreover, for each energy bin of the incoming spectrum, how a large fraction of photons that will be recorded in preferably all different bins in a recorded spectrum is calculated, as well as how many photons that are not counted and the distribution of signals in the neighboring sensor. The method further includes the step of calculating the distribution of real photon energies that belong to a recorded energy bin, wherein the weight belongs to a certain bin being the convolution of said distribution and theoretical weight factors.
In one embodiment a weight curve for trapped photon charges is computed, which further includes a distribution of signals, e.g., through simulation, from each energy bin in the photon spectrum, which also includes trapping, and through backwards calculation, calculating for each bin in a detected pulse amplitude spectrum how the photons that contributed to the bin are distributed in its energy spectrum.
In a preferred embodiment the method further includes the steps of irradiating an object through an x-ray source, detecting the beam having a spatial object information and a spectrum filtered by the object by means of said sensors, wherein the signal from the sensors is a signal deriving from a detected photon, signal from each x-ray detector for each sensor being readout and an amplitude of the signal being compared to a threshold level, generating an output for signals above or below said threshold value as an output for an amount of time, which relates to the time the signal is above or below the threshold levels, and if the signal from an x-ray photon is shared between two sensors and triggers an adjacent comparator, the two comparators outputs generating a simultaneous signal. The method further includes the step of detecting the simultaneous signal and initiating a charge sharing, indicating if the amplitude of a signal is high or low, for indicating if the x-ray photon is high energy or low energy, and counting the number of photons through the indication representing spectrums of signal amplitudes for each image pixel.
A preferred arrangement for detecting and counting photons in an x-ray beam includes a detector arrangement having sensors, and at least to each sensor coupled amplification unit, comparator means, logic unit and counter. Preferably, the counter comprises a first and a second counter for each sensor and the first and second counters correspond to high and low energy photons, respectively. Moreover, the comparator is connected to a threshold value and if the signal exceeds a predetermined threshold level the output of a comparator is a logical signal as output for an amount of time, which relates to the time, the signal exceeds the threshold levels. If the signal from an x-ray photon is shared between two sensors and two adjacent comparators, the signal generated is same at simultaneously. Additionally, simultaneous signals are detected by the logic units and if at least two adjacent comparators have same signals charge sharing is indicated. Moreover, it includes interface means for connection to further processing means.